The present invention relates generally to telecommunication techniques. More particularly, the present invention provides a method and a circuit for block undesired direct current (DC) out of input RF signals of AC-coupled broadband communication circuits while introducing a low cut-off frequency independently.
Over the last few decades, the use of communication networks exploded. In the early days Internet, popular applications were limited to emails, bulletin board, and mostly informational and text-based web page surfing, and the amount of data transferred was usually relatively small. Today, Internet and mobile applications demand a huge amount of bandwidth for transferring photo, video, music, and other multimedia files. For example, a social network like Facebook processes more than 500 TB of data daily. With such high demands on data and data transfer, existing data communication systems need to be improved to address these needs.
Over the past, AC-coupled high data rate broadband communication has been widely implemented via optical network, in which data signals are carried by laser light that is specifically modulated using various kinds of electro-optic modulators. Through various broadband communication circuits for transmitting the AC-coupled high-data rate signals, a DC blocking circuit is typically applied in many applications to inject/remove undesired input direct current (DC) or voltage in RF circuits without affecting the RF signal through the main transmission path. But designing a DC blocking circuit that provides a controllable bias (voltage and current) while independently and simultaneously defining a low cut-off frequency is challenging. Conventional designs of the DC blocking circuit usually are implemented with trade-offs in reduced voltage headroom, extra die area, unstable low cut-off frequency response. For example, a common approach is the use of a blocking capacitor and resistors (two resistors in series from the supply voltage to ground) with values selected to set a DC voltage for the internal circuit. The operation frequency range of the circuit depends on the capacitor and resistors values. To achieve broadband operation the capacitor and resistors need large values and consequently can occupy a large area. Furthermore, manufacturing process variation of the resistors changes the low-frequency cut-off. Alternatively, another approach uses a blocking capacitor in combination with a current source that pulls (or pushes) current through a resistor tied to the supply (or ground) to control the circuit bias voltage. The low-frequency cut-off depends on the resistor value. A lower cut-off requires a larger resistor, but the maximum value of the resistor is limited by the supply voltage and the voltage drop when conducting the bias current.
Therefore, an improved DC blocking circuit design is desired for isolating undesired DC signal out of AC-coupled high data-rate RF signals and defining a low cut-off frequency independently of the bias voltage and current.